The present invention relates to injection molding machines and methods and, in particular, to an injection molding system that allows for injecting, cooling, and ejecting plastic components in multi-shot injection molding of multicolored or multi-material parts.
Injection molding is a manufacturing process in which heated thermoplastic is forced under pressure into a mold. After the thermoplastic cools, the mold is separated along a part line and a molded thermoplastic part is ejected. With the proper mold, complex parts can be manufactured in extremely high volumes and low per-piece costs.
Many products that cannot be manufactured by injection molding in a single mold can be assembled from one or more separately molded parts. The step of assembling these parts can significantly increase the cost of the product and in certain cases decrease part volumes otherwise obtainable.
For this reason, there is considerable interest in so-called “in-mold” assembly techniques. In one such technique termed “two-shot” molding, a mold having replaceable portions allows different features to be added into one changing mold cavity over several sequential steps of plastic injection. The resulting product may be a single, fused structure or, by making the two shots of plastics that resist adhesion to each other, the resulting product may be an assembled collection of movable parts.
In conventional two-shot molding, portions of the mold are rotated. To produce a two-shot plastic part, first one shot of material is injected into a portion of the mold at a first molding station, the mold then opens and rotates portions of the mold 180° carrying parts to a second molding station, and the mold closes again. A second shot is then injected around the first shot at the second molding station to create a plastic part with two colors or materials. Simultaneously, the first shot is injected again at the first molding station. When the mold opens this time, the complete part is ejected at the second molding station. The mold will then rotate and close to repeat the cycle again. The rotary mold technique permits parallel simultaneous injection at the first and second molding stations of both shots. This results in relatively short cycle times, so that production is optimized.
To further reduce cycle times, it is known to permit simultaneous injecting and ejecting of plastic components, as described in U.S. Pat. No. 6,790,027 assigned to the assignee of the present invention and incorporated herein by reference. This injection mold provides a three-position, rotary indexing plate assembly which permits simultaneous injecting and ejecting of plastic components. The three stations of injection positions and ejection position are spaced apart in 120° increments and are in a plane perpendicular to the axis along which the molds separate. Core pins, forming the movable part of the molds, rotate along an axis parallel to the separation axis of the molds are also spaced apart in 120° increments and are in the same plane as the mold and ejection positions and rotate to carry parts between the mold positions.
This molding technique allowing simultaneous two-shot injecting and ejecting provides only limited cooling in between the time of the second injection until the ejection of the finished product as determined by the time it takes to open the mold, rotate the parts carriers, and close the mold. If additional cooling time is required, the ejection may be delayed until after injection of the subsequent first and second shots, but this negatively impacts cycle time, hence throughput and production efficiency.
A significant limitation to this technique described above of multi-station multi-shot injection, is that increasing the number of parts carriers required a significant increase in the mold size. Conventional injection molding machines may not be large enough to house such a mold and may not exert enough pressure on such a mold.